An omnidirectional antenna, called a non-directional antenna, is an antenna designed to radiate electromagnetic waves uniformly all around 360 degrees in the horizontal direction. In a mobile communication network, it is impossible to predict a direction where a mobile communication terminal moves due to a characteristic thereof. Thus, the mobile communication terminal is typically provided with an omnidirectional antenna that employs a circular monopole antenna structure. An antenna installed in a mobile communication network base station or relay station is usually provided with a directional antenna for directing each service range divided into three sectors.
Recently, as the Long Term Evolution (LTE) service has become more popular, it is required to construct a small cell or an ultra-small cell equipment for ensuring a smooth service in a shaded area, such as the inside of a building, and also for increasing a data transmission speed. Small cells for outdoor use are serviced at a coverage of 0.5 to 1.5 km, and a small size is also required for the equipment itself. Thus, as an antenna applied to the corresponding equipment, it may be useful to adopt an omnidirectional antenna.
A commonly used omnidirectional antenna mainly uses single polarization (V-pol). However, a MIMO (Multi Input Multi Output) technology is inevitable for the LTE service, and a dual polarization antenna is indispensable for this purpose. In an omnidirectional antenna, a conventional dual polarization means an H-polarization (H-pol; 0 degrees) and a vertical polarization (V-pol; 90 degrees).
However, a +/−45 degree dual polarization has the lowest correlation between the two polarizations in terms of the reflection or diffraction of radio waves due to fading. Thus, a directional antenna, which is usually applied to the base station or the relay station, mainly uses the +/−45 degree dual polarization. Accordingly, studies have been made to generate the +/−45 degree dual polarization even in an omnidirectional antenna. However, it is difficult to implement a structure for generating the +/−45 degree dual polarization while satisfying omni-directionally even radiation characteristics. Furthermore, it is more difficult to implement an omnidirectional antenna in a small size in consideration of the fact that it is installed in a small cell, such as inside a building, while generating the +/−45 degree dual polarization.